OpenJPA enhances the specification's support for persistent fields in many ways.
This section documents aspects of OpenJPA's persistent field handling that may
affect the way you design your persistent classes.

6.1.
Restoring State

While the JPA specification says that you should not use rolled back objects,
such objects are perfectly valid in OpenJPA. You can control whether the
objects' managed state is rolled back to its pre-transaction values with the
openjpa.RestoreState configuration property. none does not roll back state
(the object becomes hollow, and will re-load its state the next time it is
accessed), immutable restores immutable values (primitives,
primitive wrappers, strings) and clears mutable values so that they are reloaded
on next access, and all restores all managed values to their
pre-transaction state.

6.2.
Typing and Ordering

When loading data into a field, OpenJPA examines the value you assign the field
in your declaration code or in your no-args constructor. If the field value's
type is more specific than the field's declared type, OpenJPA uses the value
type to hold the loaded data. OpenJPA also uses the comparator you've
initialized the field with, if any. Therefore, you can use custom comparators on
your persistent field simply by setting up the comparator and using it in your
field's initial value.

Example 5.11.
Using Initial Field Values

Though the annotations are left out for simplicity, assume
employeesBySal and departments are persistent
fields in the class below.

public class Company {
// OpenJPA will detect the custom comparator in the initial field value
// and use it whenever loading data from the database into this field
private Collection employeesBySal = new TreeSet(new SalaryComparator());
private Map departments;
public Company {
// or we can initialize fields in our no-args constructor; even though
// this field is declared type Map, OpenJPA will detect that it's
// actually a TreeMap and use natural ordering for loaded data
departments = new TreeMap();
}
// rest of class definition...
}

6.3.
Calendar Fields and TimeZones

OpenJPA's support for the java.util.Calendar type will
store only the Date part of the field, not the
TimeZone associated with the field. When loading the date
into the Calendar field, OpenJPA will use the
TimeZone that was used to initialize the field.

Note

OpenJPA will automatically track changes made via modification methods in fields
of type Calendar, with one exception: when using Java
version 1.3, the set() method cannot be overridden, so
when altering the calendar using that method, the field must be explicitly
marked as dirty. This limitation does not apply when running with Java version
1.4 and higer.

6.4.
Proxies

At runtime, the values of all mutable second class object fields in persistent
and transactional objects are replaced with implementation-specific proxies. On
modification, these proxies notify their owning instance that they have been
changed, so that the appropriate updates can be made on the datastore.

6.4.1.
Smart Proxies

Most proxies only track whether or not they have been modified. Smart proxies
for collection and map fields, however, keep a record of which elements have
been added, removed, and changed. This record enables the OpenJPA runtime to
make more efficient database updates on these fields.

When designing your persistent classes, keep in mind that you can optimize for
OpenJPA smart proxies by using fields of type java.util.Set
, java.util.TreeSet, and
java.util.HashSet for your collections whenever possible. Smart
proxies for these types are more efficient than proxies for
Lists. You can also design your own smart proxies to further
optimize OpenJPA for your usage patterns. See the section on
custom proxies for
details.

6.4.2.
Large Result Set Proxies

Under standard ORM behavior, traversing a persistent collection or map field
brings the entire contents of that field into memory. Some persistent fields,
however, might represent huge amounts of data, to the point that attempting to
fully instantiate them can overwhelm the JVM or seriously degrade performance.

OpenJPA uses special proxy types to represent these "large result set" fields.
OpenJPA's large result set proxies do not cache any data in memory. Instead,
each operation on the proxy offloads the work to the database and returns the
proper result. For example, the contains method of a
large result set collection will perform a SELECT COUNT(*)
query with the proper WHERE conditions to find out if the
given element exists in the database's record of the collection. Similarly, each
time you obtain an iterator OpenJPA performs the proper query using the current
large result set settings, as
discussed in the JDBC chapter. As you
invoke Iterator.next, OpenJPA instantiates the result
objects on-demand.

You can also add and remove from large result set proxies, just as with standard
fields. OpenJPA keeps a record of all changes to the elements of the proxy,
which it uses to make sure the proper results are always returned from
collection and map methods, and to update the field's database record on commit.

The field must be declared as either a java.util.Collection
or java.util.Map. It cannot be declared as
any other type, including any sub-interface of collection or map, or any
concrete collection or map class.

6.4.3.
Custom Proxies

OpenJPA manages proxies through the
org.apache.openjpa.util.ProxyManager interface. OpenJPA
includes a default proxy manager, the
org.apache.openjpa.util.ProxyManagerImpl (with a plugin alias name
of default), that will meet the needs of most users. The
default proxy manager understands the following configuration properties:

AssertAllowedType: Whether to immediately throw an exception
if you attempt to add an element to a collection or map that is not assignable
to the element type declared in metadata. Defaults to false.

The default proxy manager can proxy the standard methods of any
Collection, List,
Map, Queue,
Date, or Calendar class,
including custom implementations. It can also proxy custom classes whose
accessor and mutator methods follow JavaBean naming conventions. Your custom
types must, however, meet the following criteria:

Custom container types must have a public no-arg constructor or a public
constructor that takes a single Comparator parameter.

Custom date types must have a public no-arg constructor or a public
constructor that takes a single long parameter
representing the current time.

Other custom types must have a public no-arg constructor or a public copy
constructor. If a custom types does not have a copy constructor, it must be
possible to fully copy an instance A by creating a new instance B and calling
each of B's setters with the value from the corresponding getter on A.

If you have custom classes that must be proxied and do not meet these
requirements, OpenJPA allows you to define your own proxy classes and
your own proxy manager. See the openjpa.util package
Javadoc for details on the interfaces involved,
and the utility classes OpenJPA provides to assist you.

You can plug your custom proxy manager into the OpenJPA runtime through the
openjpa.ProxyManager configuration property.

Example 5.14.
Configuring the Proxy Manager

<property name="openjpa.ProxyManager" value="TrackChanges=false"/>

6.5.
Externalization

OpenJPA offers the ability to write
custom field mappings in
order to have complete control over the mechanism with which fields are stored,
queried, and loaded from the datastore. Often, however, a custom mapping is
overkill. There is often a simple transformation from a Java field value to its
database representation. Thus, OpenJPA provides the externalization service.
Externalization allows you to specify methods that will externalize a field
value to its database equivalent on store and then rebuild the value from its
externalized form on load.

Note

Fields of embeddable classes used for @EmbeddedId values in
JPA cannot have externalizers.

The OpenJPA
org.apache.openjpa.persistence.Externalizer
annotation sets the name of a method that will be invoked to convert
the field into its external form for database storage. You can specify either
the name of a non-static method, which will be invoked on the field value, or a
static method, which will be invoked with the field value as a parameter. Each
method can also take an optional
StoreContext parameter for access to a persistence context.
The return value of the method is the field's external form. By default, OpenJPA
assumes that all named methods belong to the field value's class (or its
superclasses). You can, however, specify static methods of other classes using
the format <class-name>.<method-name>.

Given a field of type CustomType that externalizes to a
string, the table below demonstrates several possible externalizer methods and
their corresponding metadata extensions.

The OpenJPA
org.apache.openjpa.persistence.Factory annotation
contains the name of a method that will be invoked to instantiate the field from
the external form stored in the database. Specify a static method name. The
method will be invoked with the externalized value and must return an
instance of the field type. The method can also take an optional
StoreContext parameter for access to a persistence context.
If a factory is not specified, OpenJPA will use the constructor of the field
type that takes a single argument of the external type, or will throw an
exception if no constructor with that signature exists.

Given a field of type CustomType that externalizes to a
string, the table below demonstrates several possible factory methods and their
corresponding metadata extensions.

If your externalized field is not a standard persistent type, you must
explicitly mark it persistent. In OpenJPA, you can force a persistent field
by annotating it with
org.apache.openjpa.persistence.Persistent annotation.

Note

If your custom field type is mutable and is not a standard collection, map, or
date class, OpenJPA will not be able to detect changes to the field. You must
mark the field dirty manually, or create a custom field proxy.
See
OpenJPAEntityManager.dirty for how to mark a
field dirty manually in JPA.
See Section 6.4, “
Proxies
” for a discussion of proxies.

You can externalize a field to virtually any value that is supported by
OpenJPA's field mappings (embedded relations are the exception; you must declare
your field to be a persistence-capable type in order to embed it). This means
that a field can externalize to something as simple as a primitive, something as
complex as a collection or map of entities, or anything in
between. If you do choose to externalize to a collection or map, OpenJPA
recognizes a family of metadata extensions for specying type information for the
externalized form of your fields - see Section 3.2.6, “
Type
”. If the
external form of your field is an entity object or contains entities, OpenJPA
will correctly include the objects in its persistence-by-reachability
algorithms and its delete-dependent algorithms.

You can query externalized fields using parameters. Pass in a value of the field
type when executing the query. OpenJPA will externalize the parameter using the
externalizer method named in your metadata, and compare the externalized
parameter with the value stored in the database. As a shortcut, OpenJPA also
allows you to use parameters or literals of the field's externalized type in
queries, as demonstrated in the example below.

Note

Currently, queries are limited to fields that externalize to a primitive,
primitive wrapper, string, or date types, due to constraints on query syntax.

Example 5.16.
Querying Externalization Fields

Assume the Magazine class has the same fields as in the
previous example.

// you can query using parameters
Query q = em.createQuery("select m from Magazine m where m.url = :u");
q.setParameter("u", new URL("http://www.solarmetric.com"));
List results = q.getResultList();
// or as a shortcut, you can use the externalized form directly
q = em.createQuery("select m from Magazine m where m.url = 'http://www.solarmetric.com'");
results = q.getResultList();

6.5.1.
External Values

Externalization often takes simple constant values and transforms them to
constant values of a different type. An example would be storing a
boolean field as a char, where true
and false would be stored in the database as
'T' and 'F' respectively.

OpenJPA allows you to define these simple translations in metadata, so that the
field behaves as in full-fledged
externalization without requiring externalizer and factory methods.
External values supports translation of pre-defined simple types (primitives,
primitive wrappers, and Strings), to other pre-defined simple values.

Use the OpenJPA
org.apache.openjpa.persistence.ExternalValues
annotation to define external value translations. The values are
defined in a format similar to that of
configuration plugins, except that the value pairs represent Java and
datastore values. To convert the Java boolean values of true
and false to the character values T and
F, for example, you would use the extension value:
true=T,false=F.